David Cameron also offered his congratulations to Gen Sir David Richards on his appointment and thanked Sir Jock.

"Sir Jock has served with real distinction in his time as CDS. I have been personally grateful for his advice since becoming Prime Minister and know that he will continue to make an extremely valuable contribution in the months ahead," he said.

"I have no doubt that Sir David will build on this in the years to come, ensuring that all three services play their part in protecting Britain's national security in Afghanistan and elsewhere."

Dr Fox disclosed last month that Air Chief Marshal Stirrup - who had been due to continue to April next year - would be stepping down early.

The appointment of Gen Richards as his successor will be seen as a signal of the priority the Government attaches to the war in Afghanistan.

He has served as the head of the international coalition military force in the country, making him the first British general to command American troops since the Second World War. He said afterwards that the country and its people had "entered my bloodstream".